Restriction of a field-of-view (FOV) provides a number of advantages in magnetic resonance imaging (MRI) when imaging structures are disposed deeply within a subject's body and surrounded by a large volume of tissue. A reduction of the FOV in the phase-encoding direction can help to reduce scan time, avoid artifacts, and increase spatial resolution. Cardiovascular imaging may benefit from a reduction of the FOV, particularly in applications to the heart and carotid arteries.
Current techniques for reduced FOV imaging can be broadly grouped into two classes: preparative sequences for suppression of unwanted regions and selective excitation of the volume of interest. The first approach utilizes different spatial saturation schemes ranging from a single 90° pulse to multi-pulse, variable flip angle sequences. The second class of technical solutions for the FOV reduction includes the variants of inner volume imaging by using a spin-echo sequence, with excitation and refocusing pulses applied to orthogonal planes and two-dimensional (2-D) spatially selective excitation pulses. Each of these techniques has some limitations, which motivate the design of an alternative approach.
For example, spatial saturation is known to suffer from high sensitivity to RF in-homogeneity and poor performance for media with short T1, although more complicated solutions based on a long train of low-flip-angle adiabatic pulses have demonstrated marked improvements. A spin-echo (or fast spin-echo (FSE)) sequence with orthogonal excitation and refocusing is incompatible with multislice acquisition, since refocusing pulses in such a sequence will inevitably saturate neighboring slices. This limitation makes this technique suitable only for a single-slice or three-dimensional (3-D) imaging. Spatially-selective 2-D RF pulses have an inherently periodic excitation profile with side lobes, which may excite the magnetization outside the observed FOV and produce aliasing artifacts, making the design of such pulses a complicated problem and imposing limitations on the degree of the FOV reduction dependent on the object size and pulse parameters. In addition, 2-D pulses are long and, therefore, sensitive to the effects of T2 decay.
A specific feature of many cardiovascular applications is the need for effective blood suppression in MRI imaging, which can be achieved by employing a double-inversion-recovery (DIR) preparative sequence. In contrast, an alternative scheme utilizing saturation of inflowing blood frequently produces generally unsatisfactory results. It is also known that a classic DIR technique has a major drawback of being extremely time-inefficient, due to its inherent single-slice acquisition. Recently proposed multislice DIR methods appear to overcome this limitation, although their benefits become apparent only for long-repetition time (i.e., TR) applications, and their time performance is somewhat limited by the relatively long inversion delays and outflow requirements. Thus, a technique for reducing FOV would be desirable for improving the time efficiency of black-blood cardiovascular imaging in both single-slice and multislice implementations.
Accordingly, it would be desirable to develop a new approach for achieving reduced FOV imaging, which utilizes the principle of quadruple-inversion-recovery (QIR) and allows simultaneous suppression of unwanted signals originating from both the static parts of the object and inflowing blood. Initially developed for T1 insensitive blood signal suppression as described in a commonly assigned copending U.S. patent application Ser. No. 10/740,354, filed on Dec. 18, 2003, QIR has a specific application for black-blood contrast-enhanced imaging. It would further be desirable to develop a more general preparative sequence for QIR, which would suppress outer volume and blood contributions into the signal and enable a multislice extension of the QIR method originally proposed for single-slice acquisition.